1. Field of the Invention
The present invention relates to a thin film transistor, a manufacturing method thereof, a display device including the thin film transistor, and a manufacturing method thereof.
2. Description of the Related Art
In recent years, thin film transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Thin film transistors are widely used for electronic devices such as ICs (integrated circuits) and electro-optical devices. In particular, rapid development of thin film transistors as switching elements of image display devices typified by liquid crystal display devices, EL (electro luminescence) display devices, and the like has been demanded. In an active matrix liquid crystal display device, a voltage is applied to a pixel electrode connected to a selected switching element and an opposite electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the opposite electrode is modulated optically. The optical modulation can be recognized as a display pattern by an observer. An active matrix liquid crystal display device here means a liquid crystal display device which employs a method in which a display pattern is formed on a screen by driving pixel electrodes arranged in matrix using switching elements. An active matrix EL display device means an EL display device which employs a method in which a display pattern is formed on a screen by driving pixels arranged in matrix using switching elements.
The application range of the active matrix liquid crystal display device and the active matrix EL display device is expanding, and demands for larger screen size, higher definition, and higher aperture ratio are expanding. In addition, it is demanded that the active matrix liquid crystal display device and the active matrix EL display device have high reliability and that a production method of the active matrix liquid crystal display device and the active matrix EL display device offers high yield and reduces production cost. As a method for increasing yield and reducing production cost, simplification of the process can be given.
In an active matrix liquid crystal display device and an EL display device, thin film transistors are mainly used as switching elements. In manufacturing thin film transistors, reduction in the number of photomasks used in photolithography is important for simplification of the process. For example, if one photomask is additionally used, the following steps are further needed: resist application, prebaking, light exposure, development, postbaking, and the like and, moreover, steps before and after the aforementioned steps, such as film formation, etching, resist removal, cleaning, drying, and the like. The number of steps is significantly increased only by adding one step using a photomask in the manufacturing process. Therefore, many techniques for reducing the number of photomasks required in a manufacturing process have been developed.
Thin film transistors are broadly classified into top gate thin film transistors in each of which a channel formation region is provided below a gate electrode and bottom gate thin film transistors in each of which a channel formation region is provided above a gate electrode. It is known that the number of photomasks used in a process for manufacturing a bottom gate thin film transistor is smaller than that in a process for manufacturing a top gate thin film transistor. A bottom gate thin film transistor is generally manufactured using three photomasks.
Many conventional techniques for reducing the number of photomasks use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus. There is a possibility that the yield or the like is decreased due to various problems caused by usage of such a complicated technique. Moreover, in many cases, electric characteristics of thin film transistors are sacrificed due to the priority of the process simplification.
As typical means for reducing the number of photomasks in a manufacturing process of a thin film transistor, a technique using a multi-tone mask (called a half-tone mask or a gray-tone mask) is widely known. A technique for reducing the number of manufacturing steps by using a multi-tone mask is disclosed in Patent Document 1 (Japanese Published Patent Application No. 2003-179069), for example.